Generally, such radio apparatus are used at frequencies higher than 100 MHz so that many frequency channels can be obtained and the apparatus can be miniaturized. For example a car telephone set mounted on a motor car has been operated an 800 MHz band. For the purpose of using at such high frequency an LC oscillator has mainly been used as a source of oscillation of a motor car telephone set.
This LC oscillator is constituted by connecting an inductance to be constituted by a microstrip line or a coaxial line and a voltage variable capacitance C such as a varactor diode so as to form an oscillation circuit. In such an LC oscillator, the dimension of the inductance L is determined by the operating frequency and since the varactor diode itself has a construction difficult to integrate there is a limit for miniaturization of the oscillator.
For example, according to a paper entitled "800 MHz VCO for Mobile Radio Synthesizer" of M. Makimoto, K. Nakabe and S. Nishiki in Electron Letter Vol. 15, No. 15, pages 460 and 461, published June 1979, the dimension of the oscillator is about 17 cm.sup.3. For further miniaturizing such LC oscillator, one may consider to form the inductance L with a high dielectric material. However, the practical upper limit of the specific dielectric constant of the high dielectric material is about 40 so that at the frequency band of 800 MHz at which the car telephone set operates, .lambda./4 becomes about 1.5 cm so that when one considers the dimensions of an active circuit and the varactor diode, the dimension of the oscillator would be about 4 cm.sup.3 so that it is difficult to miniaturize it beyond this size with an oscillator of this size, when one considers other paths constituting the car telephone set such oscillator is not suitable for use as the oscillator of such smaller radio apparatus as handy type apparatus.
We have noted an oscillator constituted by a surface acoustic wave coupling element acting as a delay line that is expectable to miniaturize and an amplifier. Such oscillator is disclosed in a paper entitled "Surface acoustic wave devices and applications 6. Oscillators" by M. F. Lewis, Ultrasonics, Pages 115-123, May 1974.
This oscillator utilizes a surface acoustic wave coupling element having two dephased outputs for one input, and the two outputs, are synthesized with a variable resistor and then inputted to an input of the amplifier, and is constructed such that the oscillation frequency is varied by changing the ratio of synthesis of said variable resistor. Although this oscillator utilizes the surface acoustic wave coupling element capable of miniaturizing the LC oscillator described above, since it utilizes a mechanical variable resistor there is a limit for the miniaturization thereof. Although a PIN diode may be substituted for the variable resistor the PIN diode is too difficult to integrate so that there is also a limit for the miniaturization. It was also considered to combine the surface acoustic wave coupling element with such external phase shifter as a CR phase shifter or an LC phase shifter, in order to provide a variable characteristic for the phase shifter, it is necessary to use aforementioned varactor diode that acts as a variable capacitance which presents a problem for integration. Furthermore, where the CR phase shifter is used insertion loss increases. Further, as the LC phase shifter utilizes an inductance L it is difficult to minaturize it in the same manner as the firstly described LC oscillator.